A massive volcanic eruption that occurred in the distant past killed off
much of central India's forests and may have pushed humans to the brink
of extinction, according to a new study that adds evidence to a
controversial topic.

The
Toba eruption, which took place on the island of Sumatra in Indonesia
about 73,000 years ago, released an estimated 800 cubic kilometers of
ash into the atmosphere that blanketed the skies and blocked out
sunlight for six years. In the aftermath, global temperatures dropped
by as much as 16 degrees centigrade (28 degrees Fahrenheit) and life on
Earth plunged deeper into an ice age that lasted around 1,800 years.

In
1998, Stanley Ambrose, an anthropology professor at the University of
Illinois, proposed in the Journal of Human Evolution that the effects
of the Toba eruption and the Ice Age that followed could explain the
apparent bottleneck in human populations that geneticists believe
occurred between 50,000 and 100,000 years ago. The lack of genetic
diversity among humans alive today suggests that during this time
period humans came very close to becoming extinct.

To
test his theory, Ambrose and his research team analyzed pollen from a
marine core in the Bay of Bengal that had a layer of ash from the Toba
eruption. The researchers also compared carbon isotope ratios in fossil
soil taken from directly above and below the Toba ash in three
locations in central India — some 3,000 miles from the volcano — to
pinpoint the type of vegetation that existed at various locations and
time periods.

Heavily forested regions leave carbon isotope fingerprints that are distinct from those of grasses or grassy woodlands.

The
tests revealed a distinct change in the type of vegetation in India
immediately after the Toba eruption. The researchers write in the
journal Palaeogeography, Palaeoclimatology, Palaeoecology that their
analysis indicates a shift to a "more open vegetation cover and reduced
representation of ferns," which grow in humid conditions, all of which
"would suggest significantly drier conditions in this region for at
least 1,000 years after the Toba eruption."

The
dryness probably also indicates a drop in temperature "because when you
turn down the temperature you also turn down the rainfall," Ambrose
said. "This is unambiguous evidence that Toba caused deforestation in
the tropics for a long time."

He
also concluded that the disaster may have forced the ancestors of
modern humans to adopt new cooperative strategies for survival that
eventually permitted them to replace Neanderthals and other archaic
human species.

Although
humans survived the event, researchers have detected increasing
activity underneath a caldera at Yellowstone National Park, where some
suspect another supervolcanic eruption will eventually take place.
Though not expected to occur anytime soon, a Yellowstone eruption could coat half the United States in a layer of ash up to 3 feet (1 meter) deep.

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